This invention relates to protective DC link circuits for brushless DC motors, electronically commutated motor (ECM) systems, apparatus having an ECM therein for driving a rotatable component and a circuit for controlling the DC link when the ECM generates negative torque, and methods for protecting an ECM system.
Typically, the voltage across a motor's DC link increases as the motor slows because control systems for brushless DC motors often employ regenerative braking, dynamic braking or shunt current braking techniques. These braking techniques, and others, generate negative torque for retarding rotation of the motor's rotatable assembly. Also, braking or coasting a motor which is driving a load having a certain amount of inertia may also generate negative torque. Such negative torque supplies current back into the DC link from the energy in the load and raises the voltage across a DC link capacitor as a function of its capacitance and the current. In other words, negative torque usually increases the DC link voltage. This increase in DC link voltage requires the link capacitor as well as the power switching elements to be rated for higher voltages than is necessary when the motor operates in the positive torque state.
Electronically commutated motors are described in co-assigned U.S. Pat. Nos. 4,437,325, 4,500,821, 4,532,459, 4,540,921, 4,556,827, 4,642,536, 4,654,566 and 4,686,436, the entire disclosures of which are incorporated herein by reference. These electronically commutated motors may be advantageously employed in many different fields or motor applications among which are domestic appliances, e.g., automatic washing or laundry machines such as disclosed in the aforementioned U.S. patents. Other applications for such electronically commutated motors include heating, ventilating and air conditioning systems, refrigeration systems and automobile applications, such as disclosed in commonly assigned U.S. patent application Ser. Nos. 07/889,708, 08/023,790now abandoned and 08/108,328 now Pat. No. 5423192, the entire disclosures of which are incorporated herein by reference.